In view of recent development of electronic equipment such as portable telephones, notebook-size personal computers, electronic personal data books, etc., there are demands for production of semiconductor packages of higher density, smaller size and reduced thickness which are useful for such electronic equipment.
To meet said demands, various kinds of semiconductor packages have been developed. Examples are LSI-mounting TAB, tape carriers, plastic leaded chip carriers (PLCC), ball grid arrays (BGA), chip-size packages (CSP), flip chips, etc. These semiconductor packages have excellent features but are defective in production efficiency and mounting reliability.
Methods have been proposed for producing chip-size semiconductor packages among said semiconductor packages in an attempt to improve the production efficiency and the mounting reliability (of. Japanese Unexamined Patent Publication No. 79362/1998). According to the proposed methods, electronic circuits for a plurality of semiconductor chips are formed on a wafer and bumps are provided on the semiconductor chips. Then, after the wafer is placed into a mold cavity, a resin is supplied to the space around the bumps to encapsulate the bumps by the so-called transfer molding. Thereafter at least the tips of bumps covered with the resin layer are exposed at the surface of the resin layer. Finally the wafer with the resin layer formed thereon is cut into individual semiconductor chips to obtain semiconductor packages.
According to the foregoing conventional methods for producing semiconductor packages, a resin layer is formed on the bump-arranged surface of the wafer before mounting the semiconductor packages on the printed circuit board. Consequently the methods eliminate the need for the step of filling an encapsulation resin into a narrow space between the semiconductor chips and the printed circuit board after mounting the semiconductor chips on the board so that the mounting reliability is improved. Because of the encapsulation of the wafer with a resin, the methods can achieve a higher production efficiency than the encapsulation of individual semiconductor packages with a resin.
However, the conventional methods pose the following problems due to the use of ai  a mold. First of all, high investment in equipment is essentially needed. Further, since the encapsulating step and heat-curing step are conducted in this order within the mold, the wafer is confined in the mold for a prolonged period of time, thereby lowering the production efficiency. Moreover, it is difficult to form a resin layer having a thickness of 1 mm or less.